The proposal deals with the regulation of guanylate cyclase (GC) in the retina by a newly-discovered Ca2+-dependent activator protein (CD-GCAP). This protein stimulates rod outer segment GC (ROS-GC) at Ca2+ concentrations well above those normally found in photoreceptor cells, and is distinct from the two guanylate cyclase activating proteins (GCAPs) that stimulate GC as the Ca2+ concentration is lowered below its resting dark-adapted level. The applicant proposes that the rod outer segment GC is likely to be found in other locations in the retina, and that CD-GCAP may be the physiological regulator at these sites. To address this hypothesis, the investigator plans to: (1) fully characterize the CD-GCAP to determine Ca2+ binding stoichiometry, subunit structure, amino acid sequence, and retinal localization; (2) perform structural studies on ROS-GC and CD-GCAP to identify the precise sites of interaction between the two proteins; (3) localize ROS-GC-like cyclases in retinal synaptic layers and determine whether the pattern of immunoreactivity differs in animal models of retinal degeneration, and; (4) evaluate the consequences of CD-GCAP activation of GC and the concomitant production of cGMP in retinal synaptosomal preparations.